1. Technical Field
The present invention relates to a physical quantity sensor and a physical quantity measuring method for measuring displacement and velocity of an object based on interference information generated by a self-coupling effect of laser light emitted from a semiconductor laser and return light from the object.
2. Related Art
In a frequency modulated continuous wave (FMCW) radar, a standing wave radar, a self-mixing type laser sensor or the like, a displacement (velocity) measuring method using the interference principle generally uses signal processing such as a Fast Fourier Transform (FFT), counting processing of interference patterns or the like to calculate displacement or velocity of a measurement target based on beats or a frequency of interference patterns. However, data having long sampling period and high sampling frequency are required to achieve high resolution using the FFT, which may require an enormous amount of processing time. In addition, in the counting processing of interference patterns, a sensor needs to be physically vibrated or an analysis of an amplitude of the interference patterns needs to be made to have a measure displacement smaller than a half wavelength, which results in a limitation on vibration measurement which is periodical movement of a measurement object and the counting processing of interference patterns takes a long time.
The present inventors have suggested a wavelength modulation-typed laser measuring instrument using a self-coupling effect of a semiconductor laser (see e.g., JP-A-2006-313080). FIG. 20 shows a configuration of this laser measuring instrument. The laser measuring instrument of FIG. 20 includes: a semiconductor laser 201 which emits laser light toward an object 210; a photodiode 202 which converts light output from the semiconductor laser 201 into electrical signals; a lens 203 which collects the light from the semiconductor laser 201 to direct the light toward the object 210, and also collects return light from the object 210 to direct the light toward the semiconductor laser 201; a laser driver 204 which drives the semiconductor laser 201 to alternately switch between a first oscillation period during which an oscillation wavelength continuously increases and a second oscillation period during which the oscillation wavelength continuously decreases; a Transimpedance AMPLIFIER 205 which converts an output current from the photodiode 202 into a voltage and amplifies the voltage; a signal extracting circuit 206 which twice differentiates an output voltage of the Transimpedance AMPLIFIER 205; a counting circuit 207 which counts the number of mode hop pulses (MHPs) included in an output voltage of the signal extracting circuit 206; a computing device 208 which calculates a distance to the object 210 and a velocity of the object 210; and a display device 209 which displays a calculation result of the computing device 208.
The laser driver 204 supplies a triangular wave driving current as an injection current, which is repeatedly increased or decreased at a certain variation rate with respect to time, to the semiconductor laser 201. This allows the semiconductor laser 201 to be driven to alternately switch between a first oscillation period during which an oscillation wavelength continuously increases at a certain variation rate and a second oscillation period during which the oscillation wavelength continuously decreases at a certain variation rate. FIG. 21 is a view showing a temporal change of the oscillation wavelength of the semiconductor laser 201. In FIG. 21, P1 represents the first oscillation period, P2 represents the second oscillation period, λa represents the minimum value of the oscillation wavelength for each period, λb represents the maximum value of the oscillation wavelength for each period, and Tt represents a cycle of a triangular wave.
The laser light emitted from the semiconductor laser 201 is collected by the lens 203 and is then incident onto the object 210. Light reflected from the object 210 is collected by the lens 203 and is then incident into the semiconductor laser 201. The photodiode 202 converts light output from the semiconductor laser 201 into a current. The Transimpedance AMPLIFIER 205 converts an output current from the photodiode 202 into a voltage and amplifies the voltage. The signal extracting circuit 206 twice differentiates an output voltage of the Transimpedance AMPLIFIER 205. The counting circuit 207 counts the number of mode hop pulses (MHPs) included in an output voltage of the signal extracting circuit 206 for each of the first oscillation period P1 and the second oscillation period P2. The computing unit 208 calculates a distance to the object 210 and a velocity of the object 210 based on the minimum oscillation wavelength λa of the semiconductor laser 201, the maximum oscillation wavelength λb thereof, the number of MHPs in the first oscillation period P1 and the number of MHPs in the second oscillation period P2. Such a self-coupling type laser measuring instrument can perform a displacement measurement of a resolution of about half-wavelength of the semiconductor laser 201 and a distance measurement of a resolution which is inversely proportional to the amount of wavelength modulation of the semiconductor laser 201.
Also, US2008/181354 discloses a counting correction technique.
The self-coupling type laser measuring instrument as described above can measure displacement and velocity of a measurement target with a resolution higher than that in the FMCW radar, standing wave radar, self-mixing type laser sensor and the like in the related art. However, since this self-coupling type laser measuring instrument requires a certain amount of measurement time (a half cycle of a carrier wave of oscillation wavelength modulation of a semiconductor laser in JP-A-2006-313080) for calculation of the same displacement and velocity as FFT, measurement errors occur in measurement of a measurement object whose velocity change is rapid. In addition, since there is a need to count the number of MHPs for signal processing, it is difficult to achieve a resolution less than a half wavelength of the semiconductor laser.